The myogenic precursor cells in postnatal and adult skeletal muscle (satellite cells) are situated adjacent to the myofiber. In growing animals some satellite cells proliferate, adding nuclei to myofibers. In adults these precursors are mitotically quiescent, but can reinitiate proliferative activity following a broad range of muscular stresses ranging from exercise to overt injury. Cell culture studies have identified several peptide growth factors that can regulate proliferation and differentiation of satellite cells; however, the in vivo association of satellite cells with their neighboring myofibers, which might be important in regulating myogenesis of satellite cells, is not maintained under such conditions. The present application focuses on the role of fibroblast growth factors (FGFs) in regulating activation and proliferation of satellite cells situated in their native position. Cultures of isolated rat muscle fibers are employed in many of the studies. The isolated fibers retain their basement membrane and satellite cells, providing a model for evaluating 'in situ' myogenesis of satellite cells. The long term goal of the proposal is the understanding of mechanisms controlling growth and regeneration of skeletal muscle. The overall hypothesis of the proposal is that specific FGFs regulate specific phases of satellite cell activation and proliferation, and that this program is linked to a specific pattern of expression of FGF receptors; FGFs are produced locally in growing and adult muscle, supporting satellite cell myogeneSis, but their levels are reduced in aging muscle. The specific aims of the proposal are: *To determine the roles of aFGF (FGF-1) and bFGF (FGF-2) during activation and proliferation of satellite cells on adult fibers. This will include analysis of FGF effects on the activation of mitogen- activated protein kinase and investigations of the nature of FGF contributing by the fiber itself. *To analyze the expression of the high affinity FGF receptors 1 and 4 (FGFR-1 and FGFR-4) by satellite cells during their activation and proliferation on adult fibers. Analysis will focus on mRNA expression. *To compare the role of FGF during myogenesis of satellite cells on fibers from growing, adult and old rats. Analysis will focus on the effects of aFGF and bFGF, expression of FGFR-1 and FGFR-4, and the contribution of endogenous FGF by the fibers. Methods to be used include fiber isolation and culture, monitoring the expression of specific proteins via immunofluorescence and Western blotting, and analysis of mRNA expression by Northern blotting, in situ hybridization and polymerase chain reaction. The proposed studies are likely to contribute to better understanding of muscle growth and aging, and can prove important for improving muscle rehabilitation following muscle disuse and injury.